Just Cause 2

A location is a spot on the map that is usually a military base, village, city, outpost or a similar settlement. Undiscovered locations will show up as small yellow squares on the map! To discover them you simply need to approach them. This will highlight their name and their type on the map. You need not complete the locations for them to count toward this achievement, discovering them is enough. You need only discover about 1/4 of all locations as there are 364 locations in total.

The achievement will likely be won during normal campaign progress. Should you get impatient I suggest picking up a plane or helicopter and fly low over locations. Having too much altitude will prevent you from revealing the location. You should be able to judge how low you need to go after some locations.

Although this seems like a lot of places to visit it only accounts for 1/4 of the locations. You will most likely get this by beating the game but if you needed it earlier just grab a jet from a base or buy the one from the black market and coast around the islands. It only requires you to over pass the location.

The achievement is pretty straight forward, discover 100 locations, and since there is 364 locations you'll need to discover roughly 27% of the locations, it isn't much and this is the only achievement you can get in locations found. As the others have stated in general you can get just below or higher then 100 locations whilst doing the story line (I found that I was under by a bit). The easiest way to find the locations are by air, especially through jets as they are the fastest transportation in the game and you dodge all traffic, etc. Personally I used the Blackmarket Jet (if upgraded it's very fast and agile) although if your strapped for cash just go to a military airport and steal a jet. To do this the fastest way possible I suggest using your map and your waypoints to target each village and work in 'lines' (connect the dots?), it's pretty fast and if you do it right you can locate 100 locations in no time. However, if your using a jet be careful or the planes' turning ability and your actual flying ability. A lot of the locations require you to fly quite low to the ground and if your not careful you could end up in a tree, a building, a mountain or the ground (it happens).